1. Field of the Invention
The present invention generally relates to the technical field of mobile communications, and more particularly relates to base station apparatuses, user apparatuses and methods that conduct communications using multiple antennas.
2. Description of the Related Art
In this type of technical field, research and development related to next generation mobile communications schemes are being pushed forward at a fast pace. A W-CDMA standardization body, 3GPP, is studying a long term evolution (LTE) system as a communications scheme to succeed W-CDMA, HSPDA, or HSUDA. In the LTE, an OFDMA (orthogonal frequency division multiple access) scheme is planned for downlink, while an SC-FDMA (single-carrier frequency division multiple access) is planned for uplink as radio access schemes (see Non-patent document 1, for example).
The OFDMA is a scheme which divides a frequency band into multiple narrow frequency bands (sub-carriers) and overlays data onto the respective frequency bands to transmit the overlaid data. Densely lining up the sub-carriers such that they are in an orthogonal relationship with one another on the frequency axis makes it possible to achieve high-speed transmission and increase frequency utilization efficiency.
The SC-FDMA scheme is a single-carrier transmission scheme which divides a frequency bandwidth and conducts transmission using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA scheme reduces fluctuations in transmission power, which is advantageous in reducing power consumption of terminals and widening the coverage.
The LTE is a system which communicates with multiple user apparatuses sharing one or more physical channels for both uplink and downlink. The channels shared by the multiple user apparatuses as described above are generally called shared channels. In the LTE, in particular, uplink communication is conducted in a physical uplink shared channel (PUSCH) and downlink communication is conducted in a physical downlink shared channel (PDSCH).
In a communications system using these shared channels, it is necessary to signal, for each sub-frame (1 ms in the LTE), which user apparatus the shared channel is allocated to. In the LTE, a control channel used in the signaling is called a physical downlink control channel (PDCCH) or a DL-L1/L2 control channel (PDCCH). The physical downlink control channel information includes downlink scheduling information, acknowledgement information (ACK/NACK), an uplink scheduling grant, an overload indicator, a transmission power control command bit, etc. (see Non-patent document 2, for example).
The downlink scheduling information includes, for example, information on allocating a downlink resource block (RB) for a downlink shared channel, the ID of the UE, the number of streams when multi-input multi-output (MIMO) is conducted, information on a pre-encoding vector, information on a hybrid automatic repeat request (HARQ), a modulation scheme, a data size, etc. Moreover, the uplink scheduling information includes information on an uplink shared channel, e.g., information on allocating an uplink resource, an ID of an UE, information on uplink transmission power, the modulation scheme, the data size, information on a demodulation reference signal in uplink MIMO, etc.
The MIMO scheme is a multi-antenna communications scheme in which multiple antennas are used in communications to achieve an increased speed and/or quality of a transmission signal. Transmission signal streams are duplicated and the respective duplicated streams are mixed with appropriate weights to make it possible to send signals to communications counterparty in a directivity-controlled beam. This is called a pre-encoding scheme, while a weighting index (weight) to be used is called “a pre-encoding vector” or more generally “a pre-encoding matrix”.
FIG. 1 schematically illustrates how the pre-encoding is performed. Two streams (transmit signals 1 and 2) are respectively duplicated at a duplicator into two sub-streams, in each of which sub-streams the pre-encoding vectors are multiplexed and combined, after which they are transmitted. From a viewpoint of utilizing a more appropriate pre-encoding matrix, it is preferably closed-loop pre-encoding as shown. In this case, based on a feedback from the receiver (a user apparatus), the pre-encoding matrix is adaptively controlled so as to take a more appropriate value. In the pre-encoding scheme, each stream is transmitted in a spatially different manner, so that a great advantageous effect in quality improvement for each stream may be expected. Moreover, from a viewpoint of achieving an improvement in throughput, taking into account a channel variation characteristic in the frequency-axis direction, not only applying one type of pre-encoding matrix to a whole system bandwidth, but also applying multiple pre-encoding matrices to one system bandwidth is being studied.
In an example shown in FIG. 2, one system bandwidth (for example, 10 MHz) is divided into four bandwidth regions, for each of which bandwidth regions a pre-encoding matrix is optimized. One bandwidth region may include a predetermined number (e.g., five) of resource blocks. One bandwidth may be about the same as a minimum system bandwidth such as 1.25 MHz, and may be wider or narrower than the minimum system bandwidth. Dividing a system bandwidth into a number of portions and applying a pre-encoding matrix to the respective divided bandwidth regions is described in Non-patent document 3, for example.
Non-patent document 1 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006
Non-patent document 2 R1-070103, Downlink L1/L2 Control Signaling Channel Structure Coding
Non-patent document 3 R1-071228, 3GPP TSG RAN WG1 Meeting#48 St. Louis, USA, Feb. 12-16, 2007